I'm using a song which has 4ch and would like to have each one of them separated as wav files.

Writing each channel to a separate file can be done by muting all but one of them in the "MOD Pattern Display" vis window (middle-click on the channel columns or use the keyboard shortcuts) and writing the file, and repeating that for each of the channels (with a different filename each time). You will probably want to disable auto-amp when doing that, so that the written files all have the same reference level.